Elastomer (Hyperelastic) Characterization

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Testing Elastomers for Hyperelastic Material Models in Finite Element Analysis

Hyperelastic material models do 2 basic things:

1. They capture the very large stress changes that may occur from elastomer confinement.

2. They can represent large elastic strain behaviors.

Additional models may be combined with hyperelastic material models to capture softening, viscoelastic effects, plasticity and fatigue.

 

Graph of Basic Hyperelastic Tests
Basic Hyperelastic Strain States

Simple Tension

Simple tension experiments are very popular for elastomers.

There are several standards for the testing of elastomers in tension. However, the experimental requirements for analysis are somewhat different than most standardized test methods. The most significant requirement is that in order to achieve a state of pure tensile strain, the specimen must be much longer in the direction of stretching than in the width and thickness dimensions. The objective is to create an experiment where there is no lateral constraint to specimen thinning. 

Pure Shear Strain State

The pure shear experiment used for analysis is not what most of us would expect. 

The experiment appears at first glance to be nothing more than a very wide tensile test. The objective is to create an experiment where the specimen is perfectly constrained in the lateral direction such that all specimen thinning occurs in the thickness direction. 

Finite element analysis of the specimen geometry will show that the specimen must be at least 10 times wider than the length in the stretching direction

Pure Shear Elastomer Specimen Schematic Planar
Planar Tension Specimen Schematic

This experiment is very sensitive to this ratio. A non-contacting strain measuring device is used to measure strain away from the clamp edges where the pure strain state is occurring. 

Planar tension test of elastomer with laser extensometer in an environmental chamber.
Planar Tension, "Pure Shear" Experiment

Simple Compression Strain State

The compression experiment is also a popular test for elastomers. 

When testing for analysis, pure states of strain are desired. This is especially difficult to achieve experimentally in compression. Because there is friction between the test specimen and the instrument platens, the specimen is not completely free to expand laterally during compression. Even very small friction coefficient levels such as 0.1 between the specimen and the platen can cause substantial shearing strains that alter the stress response to straining. Often, the maximum shear strain exceeds the maximum compression strain! Because the actual friction is not known, the data cannot be corrected. 

Compression of rubber specimen with laser extensometer.
Compression Test

Equal Biaxial Strain State

For incompressible or nearly incompressible materials, equal biaxial extension of a specimen creates a state of strain equivalent to pure compression. Although the actual experiment is more complex than the simple compression experiment, a pure state of strain can be achieved which will result in a more accurate material model. The equal biaxial strain state may be achieved by radially stretching a circular disc.

Finite Element Analysis of Equal biaxial test specimen of elastomer.
Analysis of Equal Biaxial Test Specimen
Elastomer equal biaxial test instrument for hyperelastic material models.
Equal Biaxial Test System

Volumetric Compression

Volumetric compression is an experiment where the compressibility of the material is examined.

In this experiment, a cylindrical specimen is constrained in a fixture and compressed. The actual displacement during compression is very small and great care must be taken to measure only the specimen compliance and not the stiffness of the instrument itself. The initial slope of the resulting stress-strain function is the bulk modulus. 

Volumetric compression test fixture for bulk modulus measurement.
Volumetric Test Fixture